Current sensor and overload current protective device comprising the same

ABSTRACT

A small, low-cost, wide-range current sensor excellent in environmental resistance and noise resistance and high in accuracy, and an application device, a DC magnetic field is applied to two magnetic elements ( 1   a,    1   b ) having a magnetic impedance effect by means of a magnet ( 3 ), while a negative feedback magnetic field is applied to both elements by means of a coil ( 2 ). The variation of the magnetic field depending on the external magnetic field applied to the magnetic elements ( 1   a,    1   b ) is detected by detection units ( 7   a,    7   b ) The difference between the output is amplified by a differential amplifier unit ( 8 ). Thus, detection is achieved in a wide range while eliminating the influence of the noise.

TECHNICAL FIELD

[0001] The present invention relates to a current sensor using amagnetic element having a magnetic impedance effect, and an overloadcurrent protective device for use with the current sensor.

BACKGROUND ART

[0002] Conventionally, a current transformer has been widely used as acurrent sensor, but its low sensitivity requires a laminated iron core,and the iron core generates magnetic saturation, thereby causing theproblem of an insufficient current detection range. The iron core alsocauses the problem of a large sensor unit.

[0003] On the other hand, there is a method in which a Hall element anda magnetoresistive element are used as current detection elements.However, since they are low in detection sensitivity, the sensitivity iscommonly improved by providing a magnet gathering core and a Hallelement or a magnetic element mounted at the gain of the magnetgathering core.

[0004] Like the current transformer, the above-mentioned method of usingthe magnet gathering core uses a core of at least 3˜4 cm and requires alarge sensor unit, and generates the magnetic saturation by the ironcore, thereby obtaining an insufficient current detection range.Furthermore, since the Hall element and the magnetoresistive elementhave large output fluctuations depending on the temperature, atemperature compensating circuit is required.

[0005] A high-sensitive magnetic detection element for replacing theHall element and the magnetoresistive element can be, for example, amagnetic impedance element of an amorphous wire disclosed by JapanesePatent Application Laid-open No. Hei 6-347489, and a thin film disclosedby Japanese Patent Application Laid-open No. Hei 8-73835.

[0006] A magnetic impedance element of any shape indicates ahigh-sensitive magnetic detection characteristic, but the magneticimpedance element of an element itself has nonlinearity like the exampleof the magnetic impedance characteristic by the amorphous wire shown inFIG. 10. By adding the bias magnetic field, the linearity of thedependence on the magnetic field to which an impedance variation isapplied is improved (Japanese Patent Application Laid-open No. Hei6-176930), a negative feedback coil is wound around the magneticimpedance element, a current proportional to the voltages on both endsof the magnetic impedance element is applied to the coil, and a negativefeedback is provided, thereby obtaining an element excellent inlinearity (Japanese Patent Application Laid-open No. Hei 6-347489).

[0007] The above-mentioned bias magnetic field is normally obtained byapplying power to the coil wound around. However, in this case, twotypes of coils, that is, a bias coil and a feedback coil, are required,thereby upsizing the entire system.

[0008] Furthermore, using a wire type or a thin film type magneticimpedance element, there is the problem of variable element sensitivitydepending on the material (magnetic permeability, resistivity, etc.)used when a magnetic impedance element is produced and the variance inelement size (length, film thickness, film width, etc.)

[0009]FIG. 11 shows a common example of a detection circuit of themagnetic impedance element.

[0010] The detection circuit obtains impedance of a magnetic impedanceelement 1 by outputting through the detection circuit A and theamplification circuit B the output obtained when a high frequencycurrent passes from a high frequency current generator (OSC) 4 to themagnetic impedance element 1. At this time, the output is adjusted by avariable resistor VR.

[0011] However, to reduce the variance in element sensitivity in thecircuit, it is necessary to adjust and correct each system, therebyrequiring a larger cost. Although each system can be adjusted andcorrected, an automatic correction cannot be made. Therefore, the outputof a device varies with time depending on the variations in temperature,etc., thereby causing the problem that high precision compensationcannot be realized.

[0012] Accordingly, the object of the present invention is to measure awide current range with high precision using a small and low-cost systemwithout reducing the precision by an environmental feature or with time.

DISCLOSURE OF INVENTION

[0013] To solve the above-mentioned problems, the invention according toclaim 1 includes: two magnetic detection elements which has a magneticimpedance effect and is provided near the wiring leading a current; acurrent applying unit for applying an alternating current to both endsof the magnetic detection element; a DC bias magnetic field applyingunit for applying a DC bias magnetic field to the magnetic detectionelement; two detection units for detecting the variations in magneticfield by a current from the variations in alternating current varyingdepending on an external magnetic field applied to the magneticdetection element corresponding to the magnetic detection element; adifferential amplification unit for differentiation amplifying theoutput of the two detection units; and a negative feedback magneticfield applying unit for applying a predetermined negative feedbackmagnetic field to the magnetic detection element depending on the outputof the detection unit or the differential amplification unit.

[0014] The Invention according to claim 2 includes: two magneticdetection elements which has a magnetic impedance effect and is providednear the wiring leading a current; a current applying unit for applyingan alternating current to both ends of the magnetic detection element; aDC bias magnetic field applying unit for applying a DC bias magneticfield to the magnetic detection element; a negative feedback magneticfield applying unit for applying a negative feedback magnetic field tothe magnetic detection element; a predetermined magnetic field applyingunit for applying a predetermined magnetic field to the magneticdetection element; a switch unit for applying one of the negativefeedback magnetic field and the predetermined magnetic field to themagnetic detection element; two detection units for detecting thevariations in magnetic field by a current from the variations inalternating current varying depending on an external magnetic fieldapplied to the magnetic detection element corresponding to the magneticdetection element; and a differential amplification unit fordifferentiation amplifying the output of the two detection units,characterized in that, depending on the output of the detection unit orthe output of the differential amplification unit, a negative feedbackmagnetic field is applied to two magnetic detection elements in aperiod, a predetermined magnetic field is applied to two magneticdetection elements in another period, and a predetermined amendment ismade to the output of the differential amplification unit depending onthe output of the detection unit or the output of the differentialamplification unit of each period.

[0015] In the invention according to claim 1 or 2, the negative feedbackmagnetic field applying unit is configured by a negative feedback coilprovided near the magnetic detection element and a negative feedbackelement (invention according to claim 3).

[0016] In the invention according to claim 4, two magnetic detectionelements which has a magnetic impedance effect and is provided near thewiring leading a current; a current applying unit for applying analternating current to both ends of the magnetic detection element; a DCbias magnetic field applying unit for applying a DC bias magnetic fieldto the magnetic detection element; a negative feedback coil for applyinga negative feedback magnetic field to the magnetic detection element anda plurality of negative feedback elements; a switch unit for switchingthe plurality of negative feedback elements; two detection units fordetecting the variations in magnetic field by a current from thevariations in alternating current varying depending on an externalmagnetic field applied to the magnetic detection element correspondingto the magnetic detection element; and a differential amplification unitfor differentiation amplifying the output of the two detection units,characterized in that, depending on the output of the detection unit orthe output of the differential amplification unit, the plurality ofnegative feedback elements are selected.

[0017] In any of the inventions according to claims 1 through 4, the DCbias magnetic field can be applied by a magnet provided near themagnetic detection element (invention according to claim 5), and in theinvention according to claim 5, a non-magnetic substrate is providedwith two magnetic detection elements of thin ferromagnet film, and themagnet for applying the DC bias magnetic field and the negative feedbackcoil for applying the negative feedback magnetic field are formed bythin film (invention according to claim 6).

[0018] In any of the inventions according to claims 1 through 6, the twomagnetic detection elements can be arranged such that they can haveequal absolute values of the output to the magnetic flux generated by acurrent, and have opposite polarity (invention according to claim 7).

[0019] In the invention according to claim 8, in an overload currentprotective device, which is provided with a switch for supplying acurrent from a power source to a load or cutting it off, a currentdetector for detecting the current, and a control power source forproviding power to each unit of the device, for cutting off the currentto the load when an overcurrent occurs,

[0020] the current detector is configured by two magnetic detectionelements which has a magnetic impedance effect and is provided near thewiring leading a current; a current applying unit for applying analternating current to both ends of the magnetic detection element; a DCbias magnetic field applying unit for applying a DC bias magnetic fieldto the magnetic detection element; two detection units for detecting thevariations in magnetic field by a current from the variations inalternating current varying depending on an external magnetic fieldapplied to the magnetic detection element corresponding to the magneticdetection element; and a differential amplification unit fordifferentiation amplifying the output of the two detection units; anegative feedback magnetic field applying unit for applying apredetermined negative feedback magnetic field to the magnetic detectionelement depending on the output of the detection unit or thedifferential amplification unit.

[0021] In the invention according to claim 9, in an overload currentprotective device, which is provided with a switch for supplying acurrent from a power source to a load or cutting it off, a currentdetector for detecting the current, and a control power source forproviding power to each unit of the device, for cutting off the currentto the load when an overcurrent occurs,

[0022] two magnetic detection elements which has a magnetic impedanceeffect and is provided near the wiring leading a current; a currentapplying unit for applying an alternating current to both ends of themagnetic detection element; a DC bias magnetic field applying unit forapplying a DC bias magnetic field to the magnetic detection element; anegative feedback magnetic field applying unit for applying a negativefeedback magnetic field to the magnetic detection element; apredetermined magnetic field applying unit for applying a predeterminedmagnetic field to the magnetic detection element; a switch unit forapplying one of the negative feedback magnetic field and thepredetermined magnetic field to the magnetic detection element; twodetection units for detecting the variations in magnetic field by acurrent from the variations in alternating current varying depending onan external magnetic field applied to the magnetic detection elementcorresponding to the magnetic detection element; and a differentialamplification unit for differentiation amplifying the output of the twodetection units, and depending on the output of the detection unit orthe output of the differential amplification unit, a negative feedbackmagnetic field is applied to two magnetic detection elements in aperiod, a predetermined magnetic field is applied to two magneticdetection elements in another period, and a predetermined amendment ismade to the output of the differential amplification unit depending onthe output of the detection unit or the output of the differentialamplification unit of each period.

[0023] In the invention according to claim 8 or 9, the negative feedbackmagnetic field applying unit is configured by a negative feedback coilprovided near the magnetic detection element and a negative feedbackelement (invention according to claim 10).

[0024] In the invention according to claim 11, in an overload currentprotective device, which is provided with a switch for supplying acurrent from a power source to a load or cutting it off, a currentdetector for detecting the current, and a control power source forproviding power to each unit of the device, for cutting off the currentto the load when an overcurrent occurs,

[0025] the current detector includes: two magnetic detection elementswhich has a magnetic impedance effect and is provided near the wiringleading a current; a current applying unit for applying an alternatingcurrent to both ends of the magnetic detection element; a DC biasmagnetic field applying unit for applying a DC bias magnetic field tothe magnetic detection element; a negative feedback coil for applying anegative feedback magnetic field to the magnetic detection element and aplurality of negative feedback elements; a switch unit for switching theplurality of negative feedback elements; two detection units fordetecting the variations in magnetic field by a current from thevariations in alternating current varying depending on an externalmagnetic field applied to the magnetic detection element correspondingto the magnetic detection element; and a differential amplification unitfor differentiation amplifying the output of the two detection units,characterized in that, depending on the output of the detection unit orthe output of the differential amplification unit, the plurality ofnegative feedback elements are selected.

[0026] In any of the inventions according to claims 8 through 11, the DCbias magnetic field can be applied by a magnet provided near themagnetic detection element (invention according to claim 12), and in theinvention according to claim 12, a non-magnetic substrate is providedwith two magnetic detection elements of thin ferromagnet film, and themagnet for applying the DC bias magnetic field and the negative feedbackcoil for applying the negative feedback magnetic field are formed bythin film (invention according to claim 13).

[0027] In any of the inventions according to claims 8 through 13, thetwo magnetic detection elements can be arranged such that they can haveequal absolute values of the output to the magnetic flux generated by acurrent, and have opposite polarity (invention according to claim 14).

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1 shows the configuration of the first embodiment of thepresent invention;

[0029]FIG. 2 shows the influence of a current flowing through adjacentwiring;

[0030]FIG. 3 shows the configuration of the second embodiment of thepresent invention;

[0031]FIG. 4 shows the method for detecting the detection sensitivity inFIG. 3;

[0032]FIG. 5 shows the configuration of the third embodiment of thepresent invention;

[0033]FIG. 6 shows the current detection characteristic with theconfiguration shown in FIG. 5;

[0034]FIG. 7 is an oblique view showing an example of the structure of awire-type current detection element according to the present invention;

[0035]FIG. 8 is an oblique view showing an example of the structure of athin-film-type current detection element according to the presentinvention;

[0036]FIG. 9 shows an example of the configuration of the system appliedto an overload current protective device;

[0037]FIG. 10 is a graph showing an example of a magnetic impedancecharacteristic; and

[0038]FIG. 11 shows the circuit of the conventional technology.

BEST MODE FOR CARRYING OUT THE INVENTION

[0039]FIG. 1 shows the configuration of the first embodiment of thepresent invention.

[0040] In FIG. 1, magnetic impedance elements (also referred to simplyas MI elements) 1 a and 1 b can be wire-shaped or thin-film-shaped. Acompensation coil 2 applies negative feedback to the MI elements 1 a and1 b. A magnet 3 applies a DC bias to the MI elements 1 a and 1 b. Anoscillation unit 4 applies a DC current to the MI elements 1 a and 1 b.Buffer units 5 a and 5 b are inserted depending on the level of thecurrent output by the oscillation unit 4. Reference numerals 6 a and 6 bdenote resistors. Detector means 7 a and 7 b detect the variations inalternating current depending on the external magnetic field applied tothe MI elements 1 a and 1 b. A differential amplification unit 8amplifies the differential of the output of the detection unit. Anegative feedback element 9 supplies a current to the compensation coil2 depending on the output of the differential amplification unit 8.Wiring 10 leads a detection current.

[0041] As shown in FIG. 1, the magnetic impedance elements 1 a and 1 bare arranged such that the absolute values of the magnetic flux (Hb1,Hb2) generated by the current I flowing through the wiring 10 can beequal and the directions of the magnetic flux can be opposite, and theoutput difference is calculated by the differential amplification unit8, thereby obtaining the output proportional to the current.

[0042] The compensation coil 2 and the negative feedback element 9 applya magnetic field to the MI elements 1 a and 1 b in the direction ofdecreasing the output of the differential amplification unit 8. Thenegative feedback element 9 is normally configured by a resistor so thatthe output sensitivity for the detection current can be reducedproportional to the resistance. Therefore, the measurement precision canbe improved by optimizing the value of the negative feedback element 9depending on the measurement range.

[0043] When the current sensor as shown in 1 is used as the receivingand distributing equipment, it is necessary to eliminate the influenceof a current flowing through adjacent wiring. FIG. 2 shows the influenceof a current flowing through adjacent wiring. A current I1 flowsadjacent to a current I1

[0044] The magnetic flux generated by the currents I1 and I2 isrespectively defined as φ1 and φ2. Using the magnetic flux φ1 and φ2,the output of the difference between the two MI elements 1 a and 1 b iscalculated below. $\begin{matrix}\begin{matrix}{{{differential}\quad {output}} = {{{output}\quad {of}\quad 1a} - {{output}\quad {of}\quad 1b}}} \\{= {{S2} + {N3} - \left( {{- {S2}} + {N3}} \right)}} \\{= {2{S2}}}\end{matrix} & (1)\end{matrix}$

[0045] Thus, the current I1 can be detected without the influence of thecurrent I2 flowing through adjacent wiring 10 a.

[0046] When a uniform external magnetic field is applied as noise, twoMI elements 1 a and 1 b indicate output equal in size and sign.Therefore, the influence of noise of the external magnetic field can beremoved as in the case of the current flowing through the adjacentwiring.

[0047]FIG. 3 shows the configuration of the second embodiment of thepresent invention.

[0048] In FIG. 3, a constant current unit 91, a switch unit 92, ananalog-digital conversion unit 81, and an arithmetic control unit 82configured by a microcomputer, etc. are added to the configuration shownin FIG. 1.

[0049] With the above-mentioned configuration, a magnetic field isapplied to the output of the differential amplification unit 8 using thecompensation coil 2 and the negative feedback element 9 in the directionof decreasing the output of the differential amplification unit 8. Thearithmetic control unit 82 controls the switch unit 92 to apply aconstant current from the constant current unit 91 to the compensationcoil 2, and controls the analog-digital conversion unit 81 to detect theoutput of the differential amplification unit 8. The arithmetic controlunit 82 controls the output obtained when a constant current is appliedunder a predetermined condition to be stored as a reference value,thereby comparing the output of the analog-digital conversion unit 81with the reference value, correcting the difference from the referencevalue in the output result, and correcting the output of the apparatusby the environmental characteristic such as a temperature, etc. and acharacteristic change with time. As a result, a high-precision andenvironmental resistant current sensor.

[0050]FIG. 4 shows the method for detecting the detection sensitivity inFIG. 3.

[0051] In FIG. 4, the characteristic of the output of the sensor to anexternal magnetic field indicates the characteristic of a commonmagnetic impedance element, and an arbitrary sensor output is obtainedregardless of the direction of the magnetic field based on the zeromagnetic field.

[0052] In the case 1 shown in (a), (b), and (c) in FIG. 4, the medianvalue of the bias magnetic field indicates the zero magnetic field.Therefore, the outputs of the detector means 7 a and 7 b are equal toeach other, and the output of the differential amplification unit 8 iszero.

[0053] In the case 2 shown in (d), (e), and (f), the median value of thebias magnetic field is shifted by ΔH, the output difference between thedetector means 7 a and 7 b is ΔV, and the output of the differentialamplification unit 8 is α·ΔV (α indicates a gain of the differentialamplification unit). ΔV/ΔH is only the sensitivity of the sensor.

[0054]FIG. 5 shows the configuration of the third embodiment of thepresent invention.

[0055] As clearly shown in FIG. 5, the example is characterized by thetwo negative feedback elements 9 a and 9 b. In this example, thenegative feedback elements 9 a and 9 b and the compensation coil 2 applya magnetic field to the MI elements 1 a and 1 b in the direction ofdecreasing the output of the differential amplification unit 8 dependingon the output of the differential amplification unit 8. Since thenegative feedback elements 9 a and 9 b are normally configured byresistors as describe above, the output sensitivity to the detectedcurrent can be decreased proportional to the resistance. Therefore, thevalues of the negative feedback elements 9 a and 9 b are set dependingon the measurement range, and the switch unit 92 automatically switchesthe values based on the output of the differential amplification unit 8,thereby obtaining a high-precision current detection characteristic in awide measurement range.

[0056] In FIG. 5, two negative feedback elements are switched, but threeor more negative feedback elements can also be switched. In somemeasurement ranges, no negative feedback can be performed withoutselecting a negative feedback element.

[0057]FIG. 6 shows the current detection characteristic according to thethird embodiment.

[0058] In FIG. 6, two cases, that is, a case in which no negativefeedback is carried out, and another case in which a negative feedbackis performed using a different resistance, are shown. A wider range isused in the case where a negative feedback element is performed.

[0059]FIG. 7 is an oblique view showing an example of the structure of awire-type current detection element according to the present invention.

[0060] In FIG. 7, the reference numerals 1 a and 1 b denote MI elements.The compensation coil 2 applies a negative feedback to the MI elements.The magnet 3 applies a DC bias to the MI elements. The wiring 10 leads adetection current. A shield plate 11 cancels the influence of anexternal magnetic field. A shield plate 11 cancels the influence of anexternal magnetic field. A through hole 13 retrieves a signal.

[0061]FIG. 8 shows an example of the structure of a thin-film-typecurrent detection element according to the present invention. FIG. 8(a)is a top view, FIG. 8(b) is a sectional view.

[0062] In FIG. 8, a substrate 14 shown in (b) is a nonmagneticsubstance. The reference numerals 1 a and 1 b shown in (a) denote thethin-film MI elements. The thin-film compensation coil 2 applies anegative feedback to the MI elements. The MI elements 1 a and 1 b andthe compensation coil 2 are laid on the substrate 14 through aninsulator such as nitrogen silicide, etc. Thin-film magnets 3 a and 3 bapply a DC bias to the MI element. The wide portions on both ends of theMI elements 1 a and 1 b and the compensation coil 2 are the pads forexternal wiring. Since the substrate 14 can have dimensions of severalmillimeters, amazingly small, low-cost, and low power consumption systemcan be realized.

[0063] A system using two magnetic impedance elements is describedabove, but three or more magnetic impedance elements can be used.Furthermore, the above-mentioned 1-phase current sensor can be obviouslyreplaced with three or more required phases can be applied to thecurrent sensor when it is used for receiving and distributing equipment,etc.

[0064]FIG. 9 shows an example of the overload current protective deviceto which the above-mentioned current sensor is applied.

[0065] The reference characters R, S, and T denote power supply linesconnected to a three-phase AC power source, and are connected to a motor30 through a 3-phase contactor (switch) 20 and three power supplytransformers 50 a, 50 b, and 50 c. The current detectors 40 a, 40 b, and40 c are arranged for each phase between the 3-phase contactor (switch)20 and the three power supply transformers 50 a, 50 b, and 50 c. Thecontactor 20 has three contact points 20 a, 20 b, and 20 c are coupledby the different power supply lines R, S, and T to the motor 30 throughthe primary coils of the power supply transformers 50 a, 50 b, and 50 crespectively. The set of contact points are mechanically coupled to besimultaneously operated by the electromagnetic coil 20 d. Theelectromagnetic coil 20 d is connected to the digital output of amicrocomputer 80. An electronic overload relay 100 is formed by acontrol circuit including the microcomputer 80, the current detectors 40a, 40 b, and 40 c, the power supply transformers 50 a, 50 b, and 50 c,etc.

[0066] In this example, the current detectors 40 a, 40 b, and 40 ccomprises an MI element 400 having the MI elements 1 a and 1 b and adrive/detector 401. The output of each unit is sequentially switched bya switch 60. The output of the power supply transformers 50 a, 50 b, and50 c selected by the switch 60 is connected to the analog input of themicrocomputer 80 through a half-wave rectifier 70.

[0067] A control power source is connected from the secondary coils ofthe power supply transformers 50 a, 50 b, and 50 c to a first capacitorC0 through the diodes D0, D1, and D2. The first capacitor C0 isconnected between the positive input of a voltage adjuster 90 and theground, a capacitor C1 is connected between the positive output of thevoltage adjuster 90 and the ground, and the voltage Vcc at apredetermined level is provided as a control power source. D3, D4, andD5 are protective diodes.

Industrial Applicability

[0068] According to the present invention, the following effects can beexpected.

[0069] (1) Since the magnetic flux by a current is detected by an MIelement having a magnetic impedance effect, the magnetic saturation froma widely and currently used core does not occur. As a result, anapparatus of a wide current detection range can be provided.

[0070] (2) When a bias magnetic field and a negative feedback magneticfield are applied to an MI element to improve the linearity, the biasmagnetic field is applied from a magnet, and the negative feedbackmagnetic field is applied from a compensation coil. Therefore, ascompared with the conventional configuration in which a bias magneticfield and a negative feedback magnetic field are applied from a coil, asmaller, lower-cost, and lower power consumption apparatus can berealized. Furthermore, by optimizing the value of a negative feedbackelement depending on the measurement range, the linearity can beimproved.

[0071] (3) By arranging the two MI elements such that the absolutevalues of the magnetic flux generated by the detection currents can beequal but in the opposite directions and obtaining the differencebetween the detection units, the current can be detected without theinfluence of the disturbance magnetic field or the magnetic field of thecurrent flowing through the adjacent wiring. Therefore, the currentsensor of excellent environmental resistance without an influence ofnoise can be provided.

[0072] (4) Since a common magnetic field can be applied to an MIelement, and the sensitivity of the magnetic detection element can beautomatically detected from the output at that time, an automaticcorrection can be made although the sensitivity of an element is changedby the environmental characteristic or a change with time. Therefore, ancurrent sensor excellent in environmental resistance and change withtime can be provided.

[0073] (5) A high-precision current sensor excellent in linearity in awide measurement range can be provided by setting the resistance of aplurality of negative feedback elements depending on the measurementrange, and automatically switching the values based on the output of thedifferential amplification unit.

[0074] (6) A current sensor of high environmental resistance which isnot subject to the influence of disturbance noise from the influence ofthe variance of the sensitivity of a magnetic sensor, a position error,etc. can be provided by including a magnetic shield to cut off anexternal magnetic field.

[0075] (7) Since an MI element, a bias magnet, and a negative feedbackcoil can be formed by thin film, and a substrate can be configured withthe dimensions of several millimeters, an amazingly smaller, lower-cost,and lower power consumption apparatus can be realized. Therefore, asmall, mass-producible, and high-precision current sensor can beprovided.

[0076] (8) When the above-mentioned current sensor is applied to aoverload current protective device for controlling a power supply to aload with a current cut off when a current flowing through a conductoris detected and the value of the current exceeds a predeterminedthreshold, a small, lower-cost, high-current-detection, andhigh-linearity overload current protective device can be obtained.

1. A current sensor, comprising: two magnetic detection elements whichhas a magnetic impedance effect and is provided near wiring leading acurrent; a current applying unit applying an alternating current to bothends of the magnetic detection element; a DC bias magnetic fieldapplying unit applying a DC bias magnetic field to the magneticdetection element; two detection units detecting variations in magneticfield by a current from variations in alternating current varyingdepending on an external magnetic field applied to the magneticdetection element corresponding to the magnetic detection element; adifferential amplification unit differentiation amplifying output of thetwo detection units; and a negative feedback magnetic field applyingunit applying a predetermined negative feedback magnetic field to themagnetic detection element depending on the output of the detection unitor the differential amplification unit.
 2. A current sensor, comprising:two magnetic detection elements which has a magnetic impedance effectand is provided near wiring leading a current; a current applying unitapplying an alternating current to both ends of the magnetic detectionelement; a DC bias magnetic field applying unit applying a DC biasmagnetic field to the magnetic detection element; a negative feedbackmagnetic field applying unit applying a negative feedback magnetic fieldto the magnetic detection element; a predetermined magnetic fieldapplying unit applying a predetermined magnetic field to the magneticdetection element; a switch unit applying one of the negative feedbackmagnetic field and the predetermined magnetic field to the magneticdetection element; two detection units detecting variations in magneticfield by a current from variations in alternating current varyingdepending on an external magnetic field applied to the magneticdetection element corresponding to the magnetic detection element; and adifferential amplification unit differentiation amplifying output of thetwo detection units, characterized in that depending on the output ofthe detection unit or the output of the differential amplification unit,a negative feedback magnetic field is applied to two magnetic detectionelements in a period, a predetermined magnetic field is applied to twomagnetic detection elements in another period, and a predeterminedamendment is made to the output of the differential amplification unitdepending on the output of the detection unit or the output of thedifferential amplification unit of each period.
 3. The current sensoraccording to claim 1 or 2, characterized in that the negative feedbackmagnetic field applying unit is configured by a negative feedback coilprovided near the magnetic detection element and a negative feedbackelement.
 4. A current sensor, comprising: two magnetic detectionelements which has a magnetic impedance effect and is provided nearwiring leading a current; a current applying unit applying analternating current to both ends of the magnetic detection element; a DCbias magnetic field applying unit applying a DC bias magnetic field tothe magnetic detection element; a negative feedback coil applying anegative feedback magnetic field to the magnetic detection element and aplurality of negative feedback elements; a switch unit switching theplurality of negative feedback elements; two detection units detectingvariations in magnetic field by a current from variations in alternatingcurrent varying depending on an external magnetic field applied to themagnetic detection element corresponding to the magnetic detectionelement; and a differential amplification unit differentiationamplifying output of the two detection units, characterized in thatdepending on the output of the detection unit or output of thedifferential amplification unit, the plurality of negative feedbackelements are selected.
 5. The current sensor according to any of claims1 through 4, characterized in that the DC bias magnetic field can beapplied by a magnet provided near the magnetic detection element.
 6. Thecurrent sensor according to claim 5, characterized in that anon-magnetic substrate is provided with two magnetic detection elementsof thin ferromagnet film, and a magnet for applying the DC bias magneticfield and the negative feedback coil for applying the negative feedbackmagnetic field are formed by thin film.
 7. The current sensor accordingto any of claims 1 through 6, characterized in that the two magneticdetection elements can be arranged such that the elements can have equalabsolute values of output to magnetic flux generated by a current, andhave opposite polarity.
 8. An overload current protective device whichis provided with a switch for supplying a current from a power source toa load or cutting it off, a current detector for detecting the current,and a control power source for providing power to each unit of thedevice, for cutting off the current to a load when an overcurrentoccurs, comprising: two magnetic detection elements which has a magneticimpedance effect and is provided near wiring leading a current; acurrent applying unit applying an alternating current to both ends ofthe magnetic detection element; a DC bias magnetic field applying unitapplying a DC bias magnetic field to the magnetic detection element; twodetection units detecting variations in magnetic field by a current fromvariations in alternating current varying depending on an externalmagnetic field applied to the magnetic detection element correspondingto the magnetic detection element; a differential amplification unitdifferentiation amplifying the output of the two detection units; and anegative feedback magnetic field applying unit applying a predeterminednegative feedback magnetic field to the magnetic detection elementdepending on output of the detection unit or the differentialamplification unit.
 9. An overload current protective device, which isprovided with a switch for supplying a current from a power source to aload or cutting it off, a current detector for detecting the current,and a control power source for providing power to each unit of thedevice, for cutting off the current to the load when an overcurrentoccurs, comprising: two magnetic detection elements which has a magneticimpedance effect and is provided near wiring leading a current; acurrent applying unit applying an alternating current to both ends ofthe magnetic detection element; a DC bias magnetic field applying unitapplying a DC bias magnetic field to the magnetic detection element; anegative feedback magnetic field applying unit applying a negativefeedback magnetic field to the magnetic detection element; apredetermined magnetic field applying unit applying a predeterminedmagnetic field to the magnetic detection element; a switch unit applyingone of the negative feedback magnetic field and the predeterminedmagnetic field to the magnetic detection element; two detection unitsdetecting variations in magnetic field by a current from the variationsin alternating current varying depending on an external magnetic fieldapplied to the magnetic detection element corresponding to the magneticdetection element; and a differential amplification unit differentiationamplifying output of the two detection units, characterized in thatdepending on the output of the detection unit or the output of thedifferential amplification unit, a negative feedback magnetic field isapplied to two magnetic detection elements in a period, a predeterminedmagnetic field is applied to two magnetic detection elements in anotherperiod, and a predetermined amendment is made to the output of thedifferential amplification unit depending on the output of the detectionunit or the output of the differential amplification unit of eachperiod.
 10. The overload current protective device according to claim 8or 9, characterized in that the negative feedback magnetic fieldapplying unit is configured by a negative feedback coil provided nearthe magnetic detection element and a negative feedback element.
 11. Anoverload current protective device, which is provided with a switch forsupplying a current from a power source to a load or cutting it off, acurrent detector for detecting the current, and a control power sourcefor providing power to each unit of the device, for cutting off thecurrent to the load when an overcurrent occurs, comprising: two magneticdetection elements which has a magnetic impedance effect and is providednear the wiring leading a current; a current applying unit applying analternating current to both ends of the magnetic detection element; a DCbias magnetic field applying unit applying a DC bias magnetic field tothe magnetic detection element; a negative feedback coil applying anegative feedback magnetic field to the magnetic detection element and aplurality of negative feedback elements; a switch unit switching theplurality of negative feedback elements; two detection units detectingvariations in magnetic field by a current from variations in alternatingcurrent varying depending on an external magnetic field applied to themagnetic detection element corresponding to the magnetic detectionelement; and a differential amplification unit differentiationamplifying the output of the two detection units, characterized in that,depending on the output of the detection unit or the output of thedifferential amplification unit, the plurality of negative feedbackelements are selected.
 12. The device according to any of claims 8through 11, characterized in that the DC bias magnetic field can beapplied by a magnet provided near the magnetic detection element. 13.The device according to claim 12, characterized in that a non-magneticsubstrate is provided with two magnetic detection elements of thinferromagnet film, and a magnet for applying the DC bias magnetic fieldand a negative feedback coil for applying the negative feedback magneticfield are formed by thin film.
 14. The device according to any of claims8 through 13, characterized in that the two magnetic detection elementscan be arranged such that the elements can have equal absolute values ofoutput to magnetic flux generated by a current, and have oppositepolarity.